U.S. patent application number 13/984673 was filed with the patent office on 2014-02-27 for method for fabricating fiber products and composites.
This patent application is currently assigned to UPM-KYMMENE CORPORATION. The applicant listed for this patent is Olli Ikkala, Antti Laukkanen, Markus Samuli Nuopponen, Jan-Erik Teirfolk, Andreas Walther. Invention is credited to Olli Ikkala, Antti Laukkanen, Markus Samuli Nuopponen, Jan-Erik Teirfolk, Andreas Walther.
Application Number | 20140058077 13/984673 |
Document ID | / |
Family ID | 45999870 |
Filed Date | 2014-02-27 |
United States Patent
Application |
20140058077 |
Kind Code |
A1 |
Laukkanen; Antti ; et
al. |
February 27, 2014 |
METHOD FOR FABRICATING FIBER PRODUCTS AND COMPOSITES
Abstract
Method for fabricating fiber and film products and composites
includes: introducing an aqueous gel of nanofibrillar cellulose
into a volume of organic extraction agent miscible with water so
that the aqueous gel is kept as a separate phase and forms one or
several discrete physical entities containing the nanofibrillar
cellulose within the phase; changing water with the organic
extraction agent in said one or several discrete physical entities
of nanofibrillar cellulose; and drying the nanofibrillar cellulose
by removing the organic extraction agent from the one or several
discrete physical entities of nanofibrillar cellulose. In the
method the aqueous gel of nanofibrillar cellulose is introduced
into the volume of organic extraction agent in the form of one or
several elongated elements which form a fiber-like or ribbon-like
or film-like product after drying.
Inventors: |
Laukkanen; Antti; (Helsinki,
FI) ; Teirfolk; Jan-Erik; (Turku, FI) ;
Nuopponen; Markus Samuli; (Helsinki, FI) ; Walther;
Andreas; (Koln, DE) ; Ikkala; Olli; (Helsinki,
FI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Laukkanen; Antti
Teirfolk; Jan-Erik
Nuopponen; Markus Samuli
Walther; Andreas
Ikkala; Olli |
Helsinki
Turku
Helsinki
Koln
Helsinki |
|
FI
FI
FI
DE
FI |
|
|
Assignee: |
UPM-KYMMENE CORPORATION
Helsinki
FI
|
Family ID: |
45999870 |
Appl. No.: |
13/984673 |
Filed: |
February 8, 2012 |
PCT Filed: |
February 8, 2012 |
PCT NO: |
PCT/FI12/50121 |
371 Date: |
October 28, 2013 |
Current U.S.
Class: |
536/56 ;
264/171.25 |
Current CPC
Class: |
D01D 5/06 20130101; B82Y
30/00 20130101; D21H 11/14 20130101; Y02W 30/64 20150501; Y02W
30/648 20150501; D01F 2/00 20130101; C08L 2205/16 20130101; C08J
5/24 20130101; C08J 5/005 20130101; C08J 5/045 20130101; C08L 1/02
20130101; D21C 5/02 20130101; D21H 27/10 20130101 |
Class at
Publication: |
536/56 ;
264/171.25 |
International
Class: |
D01F 2/00 20060101
D01F002/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 10, 2011 |
FI |
20115131 |
Feb 14, 2011 |
FI |
20115137 |
Claims
1-25. (canceled)
26. Method for fabricating fiber and film products and composites,
comprising: introducing an aqueous gel of nanofibrillar cellulose
into a volume of organic extraction agent miscible with water so
that the aqueous gel is kept as a separate phase and forms one or
several discrete physical entities containing the nanofibrillar
cellulose within the phase, changing water with the organic
extraction agent in said one or several discrete physical entities
of nanofibrillar cellulose, and drying the nanofibrillar cellulose
by removing the organic extraction agent from the one or several
discrete physical entities of nanofibrillar cellulose; the method
comprising: introducing the aqueous gel of nanofibrillar cellulose
into the volume of organic extraction agent in the form of one or
several elongated elements which form a fiber-like or ribbon-like
or film-like product after drying, and/or incorporating at least
one substance into the one or several discrete physical entities
through the aqueous gel and/or through the extraction agent to make
a composite of nanofibrillar cellulose and the substance.
27. Method according to claim 26, wherein the one or several
discrete physical entities are one or several elongated elements
which form the fiber-like, ribbon-like or film-like product.
28. Method according to claim 27, wherein the strength of the one
or several elongated elements is increased by stretching.
29. Method according to claim 27, wherein the fiber-like or
ribbon-like product is processed further to yarn.
30. Method according to claim 27, wherein the fiber-like or
ribbon-like product is processed further to a 2- or 3-dimensional
structure.
31. Method according to claim 27, wherein the fiber-like or
ribbon-like product, or yarn or a 2- or 3-dimensional structure
processed from it, is incorporated in a composite as
reinforcement.
32. Method according to claim 27, wherein the film-like product is
incorporated in a composite as reinforcement.
33. Method according to claim 26, wherein the substance
incorporated in the one or several physical entities is a polymer
either as such or in the form of precursors.
34. Method according to claim 31, wherein the substance
incorporated in the one or several physical entities is a polymer
either as such or in the form of precursors and the polymer forms
polymer matrix or part of it in the composite.
35. Method according to claim 27, wherein 2- or 3-dimensional fiber
structures are formed by depositing the elongated elements in situ
in the volume of organic extraction agent to 2- or 3-dimensional
structures.
36. Method according to claim 26, wherein the substance
incorporated is an active drug molecule or an extra cellular matrix
component, serum, growth factor or protein.
37. Method according to claim 33, wherein the polymer is a
hydrophobic polymer which is dissolved in the extraction agent
volume and enters the physical entities from the extraction agent
volume.
38. Method according to claim 33, wherein the polymer is a
hydrophilic polymer which is dissolved in the aqueous gel of
nanofibrillar cellulose and is incorporated in the physical
entities when the aqueous gel is introduced into the extraction
agent volume.
39. Method according to claim 26, wherein the several physical
entities are made to particles containing the nanofibrillar
cellulose and the incorporated substance.
40. Method according to claim 39, wherein the incorporated
substance is a polymer either as such or in the form of
precursors.
41. Method according to claim 40, wherein the particles are used as
at least one starting material in melt-processing or solvent
processing and an article containing said polymer and the
nanofibrillar cellulose is manufactured by the melt processing or
solvent processing.
42. Method according to claim 41, wherein the melt processing is
extrusion moulding.
43. Method according to claim 26, wherein the organic extraction
agent is selected from a water-miscible alcohol, dioxane and
THF.
44. Method according to claim 26, wherein the nanofibrillar
cellulose is functionalized cellulose.
45. Method according to claim 44, wherein the nanofibrillar
cellulose contains aldehyde, carboxyl, carboxymethyl or cationic
groups in the cellulose molecules.
46. A fiber product, wherein it is an elongated continuous filament
or thread, which contains rearranged nanocellulose fibrils giving
structural integrity to the fiber product and stiffness (tensile
modulus) of the fiber product of at least 20 GPa.
47. Fiber product according to claim 46, wherein the tensile
strength of the fiber product is at least 250 Mpa.
48. Fiber product according to claim 47, wherein the nanocellulose
fibrils are rearranged by at least partial orientation.
49. Use of the fiber product according to claim 46 for making a 2-
or 3-dimensional structure, as reinforcement in composites, or as
carrier material for functional substances.
50. Use according to claim 49, wherein the functional substance is
an electrically conducting substance, especially a conducting
polymer.
51. Use of the fiber product according to claim 47 for making a 2-
or 3-dimensional structure, as reinforcement in composites, or as
carrier material for functional substances.
52. Use of the fiber product according to claim 48 for making a 2-
or 3-dimensional structure, as reinforcement in composites, or as
carrier material for functional substances.
53. Method according to claim 28, wherein the fiber-like or
ribbon-like product is processed further to yarn.
54. Method according to claim 28, wherein the fiber-like or
ribbon-like product is processed further to a 2- or 3-dimensional
structure.
55. Method according to claim 28, wherein the fiber-like or
ribbon-like product, or yarn or a 2- or 3-dimensional structure
processed from it, is incorporated in a composite as
reinforcement.
56. Method according to claim 28, wherein the film-like product is
incorporated in a composite as reinforcement.
57. Method according to claim 34, wherein the polymer is a
hydrophobic polymer which is dissolved in the extraction agent
volume and enters the physical entities from the extraction agent
volume.
58. Method according to claim 34, wherein the polymer is a
hydrophilic polymer which is dissolved in the aqueous gel of
nanofibrillar cellulose and is incorporated in the physical
entities when the aqueous gel is introduced into the extraction
agent volume.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method for fabricating
dried cellulose products, fiber products and composites. The
invention also relates to novel fiber products and composites.
BACKGROUND OF THE INVENTION
[0002] Fibrous architectures are among the most abundant
load-carrying materials in nature. This structural motif bridges
the length scales from the smallest peptide folding motifs to
protein materials as in collagen up to larger structural entities
like spider silk or cellulose microfibrils (P. Fratzl, R.
Weinkamer, Prog. Mater. Sci. 2007, 52, 1263; M. A. Meyers, P. Y.
Chen, A. Y. M. Lin, Y. Seki, Prog. Mater. Sci. 2008, 53, 1) The
latter are composed of highly aligned native crystalline
.beta.-D-(1-4)glucopyranose polysaccharide chains (cellulose I
crystals) where the chains are strongly intermolecularly bound via
a multitude of hydrogen bonds. These microfibrils are the main
building blocks of plants and are responsible for the mechanical
strength of wood or nutshells. Their underlying native
nanocellulose fibrils (also known as nanofibrillated cellulose
(NFC) or microfibrillated cellulose (MFC)) can be isolated via
consecutive chemical/enzymatic and homogenization treatments (I.
Siro, D. Plackett, Cellulose 2010, 17, 459; S. J. Eichhorn, A.
Dufresne, M. Aranguren, N. E. Marcovich, J. R. Capadona, S. J.
Rowan, C. Weder, W. Thielemans, M. Roman, S. Renneckar, W. Gindl,
S. Veigel, J. Keckes, H. Yano, K. Abe, M. Nogi, A. N. Nakagaito, A.
Mangalam, J. Simonsen, A. S. Benight, A. Bismarck, L. A. Berglund,
T. Peijs, J. Mater. Sci. 2010, 45, 1.) They typically consist of
highly crystalline nanoscale fibrils with diameters around 1-35 nm
and several micrometers in length. Cellulose nanocrystals,
alternatively called as nanowhiskers, are related materials, which
are shorter and rod-like due to strong acid hydrolysis.
Nanofibrillated cellulose is a remarkable emerging class of
nature-derived nanomaterial for its extraordinary mechanical
properties, combining astonishing stiffness and expected strength
with a lightweight character. It has been shown earlier that
cellulose I crystals can reach a Young's modulus of up to 136 GPa
and an expected strength in the range of a few GPa (S. Iwamoto, W.
H. Kai, A. Isogai, T. Iwata, Biomacromolecules 2009, 10, 2571; H.
Yano, J. Sugiyama, A. N. Nakagaito, M. Nogi, T. Matsuura, M.
Hikita, K. Handa, Adv. Mater. 2005, 17, 153) These properties rank
them at the top end of high-performance natural materials. As a
comparison, the stiffness of cellulose I is two to three times
higher than that for glass fibers (50-80 GPa), just above typical
titanium alloys (105-120 GPa) and it approaches that of steel (200
GPa). Strikingly, all of this is realized by a purely organic
material with a comparably low density (ca. 1.6 g/mL). This renders
cellulose nanofibrils one of the most promising building blocks for
future materials.
[0003] Furthermore, NFC is based on a natural polymer that is
abundant in nature and is renewable and degradable. Therefore,
nanofibrillar cellulose might be an interesting constituent in
structures where strength is needed.
[0004] Individual cellulose polymers have a long history in the
context of fiber production. Fibers based on dissolved and
regenerated or fully hydrolyzed cellulose and its derivatives (e.g
Rayon.TM.) are widely used for textiles or reinforcements, owing to
decades of development. However, due to their inherent strength, as
originating from the crystalline character, as not preserved in
dissolution processes, NFC based materials possess the potential to
go significantly beyond the mechanical performance of molecular
cellulose materials.
[0005] Thus, numerous trials have been made on trying to achieve
nanocomposites based on NFC and synthetic engineering plastics. The
reported experiments have shown properties lower than desired,
especially with hydrophobic thermoplastics, which would be the most
important matrix polymers. The main reason for that is the
difficult nature of the NFC: water is needed to fully disperse
pristine NFC in the nanoscale. If the water is removed during the
compounding stage, the fibers aggregate and phase separation takes
place, which will lead to poor mechanical properties.
[0006] NFC production techniques are based on grinding (or
homogenization) of aqueous dispersion of pulp fibers possibly
combined with chemical/biochemical treatments. The concentration of
NFC in dispersions is typically very low, usually around 1-5%.
After the grinding process, the obtained NFC material is a dilute
viscoelastic hydrogel. At very small concentrations, the NFC
material in water forms a viscous fluid.
[0007] Thus, there is an evident need for transforming the aqueous
NFC raw material to a structure where the water is essentially
absent and the nanocellulose fibrils are arranged so that they can
be used as structural parts in composites or as fiber-like
structures of high strength.
[0008] In order that the NFC can be used as various structural
constituents, water must be removed from the NFC hydrogel. The
fundamental problem in mechanical water removal is the high
hygroscopicity of NFC and the ability of NFC hydrogel to form a
very dense and impermeable nanoscale membrane around itself, for
example during filtration. The formed membrane hinders the
diffusion of water from the gel structure, which leads to very slow
water removal rates. However, water removal is not the only
problem, but the nanocellulose fibrils must be arranged in a
structure where their strength potential can be fully utilized.
Whenever water is removed, the nanocellulose fibrils tend to
aggregate which results in poor mechanical properties of the
product.
[0009] The article Capadona J. R. et al. A versatile approach for
the processing of polymer nanocomposites with self-assembled
nanofibre templates, Nature Nanotech. 2, 765-769 (2007) describes
gels made of nano-scale cellulose whiskers which are obtained
through acid hydrolysis of tunicate mantles. The whiskers exist
initially in aqueous dispersion and they are made to an organogel
in a sol-gel process through extraction agent exchange with a
water-miscible extraction agent, whereafter the gel is filled with
a matrix polymer by immersing the gel in a solution of the polymer
and dried. During the gel-forming step acetone was introduced on
top of the aqueous whisker dispersion without mixing the layers.
The acetone was exchanged daily and the acetone layer was gently
agitated to promote the extraction agent exchange. After some days
the acetone organogel, called a "scaffold" was obtained. The
article also reports the use of acetonitrile, ethanol, methanol,
isopropanol and tetrahydrofuran as extraction agents for making the
organogel. The gelled nanofibre scaffold was impregnated with a
polymer by immersion in a polymer solution, and the nanocomposite
was dried and compacted. Using this approach, nanocomposites with
polybutadiene and polystyrene could be fabricated with improved
mechanical properties. However, the gel forming step through the
extraction agent exchange takes typically many days. No essentially
pure NFC fiber products were presented and foreseen, and materials
were blends with low weight fraction of NFC which prohibited to
achieve high mechanical properties.
SUMMARY OF THE INVENTION
[0010] It is a purpose of the invention to provide a new method for
the production of fiber products and composites which is both fast,
applicable in industrial scale, and brings about a product where
the nanocellulose fibrils are mixed arranged in a desired degree
with other constituents of the product and/or the nanocellulose
fibrils are arranged so that the product has mechanical properties
which can be attributed to the excellent mechanical properties of
the NFC itself.
[0011] It has now been found that water can be extracted from NFC
hydrogels using a water-miscible liquid, e.g. ethanol as an
extraction agent by a practical method which reduces the drying
time and makes it possible to manufacture a variety of products
starting from the NFC hydrogel.
[0012] Simultaneously with drying, a NFC product is obtained which
is in the form of fiber or film. This fiber or film may contains
one or more other constituents mixed with the nanocellulose fibrils
in the fiber or film. These constituents are incorporated into the
hydrogel, in which case they remain in the fiber or film during the
formation, or inside the extraction agent, in which case they will
penetrate into the fiber or film while the fiber or film is in
contact with the extraction agent. Both alternatives are possible
at the same time when the fiber or film is prepared.
[0013] Fiber or film is used for making products where the strength
properties of the NFC fiber or film, combined with the properties
of possibly added constituents can be used. However, the dried NFC
product can also be an intermediate product where the NFC is mixed
with one or more other constituents. This can be used as a raw
material for other manufacturing processes where the NFC will
remain in a solid final product. One preferable application is to
use the mixture of NFC and the other constituent(s) as a building
block which, possibly mixed with other materials, can be used as a
masterbatch which brings the NFC and the other constituent(s) in
the final product. One particular application is the mixture of NFC
with a polymer. This product can be used as a masterbatch for
manufacturing polymer composites where the polymer (possibly
together with another polymer brought at the manufacturing stage)
forms the matrix and the NFC forms the reinforcement. The NFC
intermediate product is preferably in other form than fiber or
film, for example individual particles of regular or irregular
shape.
[0014] NFC hydrogel, which may be obtained directly from a
manufacturing process, is introduced into a water-miscible liquid
(extraction agent) so that it exists within the extraction agent as
discrete physical entities. If a fiber product is to be
manufactured from the NFC hydrogel, the hydrogel is introduced into
the extraction agent initially as continuous elongated
"thread"-like objects, either as one individual thread or two or
more threads in parallel, whereafter the individual thread or two
or more parallel threads can be arranged in a 2D (e.g. a mesh) or
3D pattern. By a proper technique these patterns can be formed
already in the extraction agent "in situ".
[0015] Alternatively, the hydrogel can be introduced in the
water-miscible extraction agent as continuous, wider 2-dimensional
object for manufacturing a NFC-product in the form of a film, which
has preferably constant thickness.
[0016] If the purpose is only to incorporate one or more other
constituents in the NFC without creating a fiber or film product,
for example for making an intermediate masterbatch product of
desired composition, the hydrogel is introduced in the extraction
agent in the form of discrete objects of regular or indefinite
shape.
[0017] Depending on the hydrophobic or hydrophilic nature, i.e.
water solubility, of the other constituent to be added, it is
included in the extraction agent from where it can penetrate into
the NFC, or it is included already in the NFC hydrogel prior to the
introduction of the hydrogel into the extraction agent. In the
former case the constituent can be hydrophobic, for example a
polymer which is soluble in the extraction agent for making a
NFC/polymer composite, and in the latter case the constituent is
hydrophilic. It is also possible to include one or more
constituents through the hydrogel in the structure of the NFC
product, and later one or more constituents through the extraction
agent volume, provided that the constituent(s) included through the
hydrogel do not affect the penetration of the other constituent(s)
from the extraction agent volume.
[0018] The form of the physical entities depends on the way of
supplying the NFC into the extraction agent. With a proper
technique, the water in the NFC hydrogels can be completely or
partly changed to e.g. ethanol. In the second stage of the process,
the extraction agent is removed for example in vacuum and/or
elevated temperature (elevated temperature, if used, is a
temperature higher than 25.degree. C.), and essentially dry NFC is
obtained. The drying can take place also by pressure filtration.
The total process time starting from the supply of the hydrogel
into the extraction agent and ending in obtaining the dried product
after the drying step is of the order of some hours, preferably not
longer than two hours.
[0019] In a most desired embodiment, the hydrogel is introduced in
the organic extraction agent carefully so that it remains coherent
and does not become dispersed, that is, a phase boundary of the gel
against the extraction agent volume is retained the whole time
after the contact of the hydrogel with the extraction agent.
Possible ways of introducing the hydrogel into the extraction agent
volume to create discrete physical entities include supply through
a port, for example through nozzles or a slit, or directly into the
extraction agent in larger blocks which are crumbled into smaller
entities in the extraction agent volume by agitating.
[0020] When the water is completely or partly exchanged with the
extraction agent in the extraction agent volume, the physical
entities keep their original shapes where the gel was initially
supplied to the extraction agent volume or the shapes they assumed
after mechanical reduction to smaller size in the extraction
volume, but the dimensions may change through shrinking. During
this extraction agent exchange process, the NFC nanofibrils
contained in the gel are stabilized to a coherent structure, where,
however, a certain porosity exists. Due to this porosity the
evaporation of the extraction agent or its removal in another way
is easy after the physical entities are separated from the
extraction agent volume. This porosity also allows the penetration
of other constituents from the extraction agent. Other constituents
can be introduced also after drying using the residual porosity of
the physical entities.
[0021] The organic liquid that is used for the extraction agent to
exchange with the water of the hydrogel is any liquid that is
miscible with water and preferably has moderate polarity. Suitable
extraction agents are organic liquids, preferably water-miscible
alcohols including but not limited to methanol, ethanol, and
isopropanol, as well as dioxane and THF. The exchange of the water
with the extraction agent leads to an enforcement of the hydrogen
bonds between the NFC fibrils and to mechanical stabilization of
the physical entities formed, irrespective of which form they
assume in the extarction agents. The concept could also incorporate
several extraction baths, to allow addition of several functional
molecules. Even if not the most feasible process, the later
extraction baths can also use an extraction agent which is
different from the extraction agent to which the hydrogel was
initially introduced. Water-miscibility is not a precondition for
these extraction agents, but the miscibility with the previous
extraction agent.
[0022] Preferred extraction agent is ethanol, which has low
toxicity, low heat of evaporation (904 kJ/kg vs. 2256 kJ/kg for
water) and exothermic mixing reaction with water (-777 J/mol at
25.degree. C.) which lowers the energy demand. However, if a
hydrophobic polymer is to be incorporated into the NFC from the
extraction agent volume, the choice of the extraction agent may be
dependent on the polymer because the polymer must be soluble in the
extraction agent.
[0023] During the introduction of the hydrogel into the extraction
agent, the stirring or agitation, if used, must be carefully
applied to avoid the dispersion of the NFC hydrogel into the
extraction agent so that the hydrogel is maintained as discrete
physical entities.
[0024] According to one embodiment of the invention, the physical
entities obtained are elongated fiber-like structures, threads or
filaments, which can be Ipost-treated, for example for dimensional
stability. Post-treatment can be covalent cross-linking and/or
protective coating. These threads or filaments can contain, mixed
with the NFC, one or more constituents which provide some
functionality, such as antibacterial agents and/or dyes. The
threads or filaments obtained are fiber products which form in
their final application a constructional part affording strength,
either alone or as part of a larger structure. They can be
processed further to other textile structures, like spun to yarns
or woven or knitted or laid in some other methods (such as
spunbonding) to two or three-dimensional structures, or they can be
used as reinforcements in components, for example in a polymer
matrix. The two- or three-dimensional structures can also be used
as reinforcements in some applications. If the other constituent
included in these threads or filaments is a polymer, it can form a
polymer matrix or part of it in the final composite product and the
NFC can constitute the reinforcement. Such elongated composite
threads or filaments, or yarns or 2- or 3-dimensional structures
processed form the composite threads or filaments, can be used
either alone or together with additional polymer in manufacturing
the final product. The threads or filaments can be post drawn as
known for other materials in the fiber-spinning processes to allow
further alignment of the constituent NFC nanofibrils. By drawing,
known also as "stretching", the threads or the filaments can be
straightened and their strength can be increased further.
[0025] If the physical entity is a wider film, it can be treated
analogously to the threads or filaments, and it can be used as a
constructional part in the final application. As the threads or
filaments, the film can be provided with other constituents
analogously. These other constituents can serve as matrix in the
final product, i.e. the film can be a composite film containing
polymer which forms polymer matrix or part of it in the final
composite product.
[0026] According to another embodiment, the threads or filaments
formed are not used as constructional parts but they are
comminuted, either already in the extraction agent volume or after
drying, to smaller particles which can be starting material for
another manufacturing process. In this case the constituent mixed
or incorporated with the NFC may be a polymer which will form the
matrix when the particles are used as masterbatch in preparation of
a composite product by melt processing, for example by extrusion,
as well as in solvent processing. An additional amount of matrix
polymer, which can be the same polymer as in the NFC or a different
polymer compatible with it, can be incorporated in the melt
processing or solvent processing to the composite product. The
threads or filaments are not necessarily the precursors of these
particles because they can be formed of physical entities of
another shape introduced into the extraction agent volume.
[0027] According to another embodiment, the extraction agent
exchange process is used to load NFC threads or filaments or
comminuted NFC particles with hydrophobic or hydrophilic molecules.
The loaded molecules are encapsulated into the nanoporous NFC
network and the release profile of these molecules can be adjusted
by altering the alignment of the microfibrils and the porosity of
the NFC matrix.
[0028] According to another embodiment, two- or three-dimensional
fiber product is formed in situ in the extraction agent volume by
supplying the elongated fiber-like structure from a nozzle, which
is operated so that the filament is deposited in the extraction
agent according to a 2D or 3D predetermined patterns. There can be
several nozzles operating at the same time. Various fiber mats,
nonwovens and 3D scaffolds can be created by this process.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] The invention will be explained in the following with
reference to the enclosed drawings, where
[0030] FIGS. 1a and 1b are schemes of a process where the method
according to the invention is employed;
[0031] FIG. 2 is a graph showing water content of ethanol in course
of extrusion of NFC in accordance with the invention;
[0032] FIG. 3a illustrates the preparation of macroscopic NFC
fibers based on cellulose nanofibril hydrogels. The inset shows a
photograph of a real extrusion process of a dye-stained
dispersion,
[0033] FIG. 3b SFM characterization of individual cellulose
nanofibrils. The inset shows the height profile of the red
section,
[0034] FIG. 4a is a photograph of a 1 m long NFC fiber, FIG. 4b is
its optical micrograph without crossed polarizers, FIG. 4d is its
optical micrograph with crossed polarizers, FIG. 4c shows a
non-woven fiber mat prepared via consecutive extrusion, FIGS. 4e-4h
are SEM images from the micro- to the nanoscale,
[0035] FIGS. 5a to 5d show mechanical properties, optical
transparency, wettability control and encapsulation/release of
model compounds for NFC fibers,
[0036] FIGS. 6a and 6b show some advanced functionalities of NFC
fibers, such as electric conductivity (FIG. 6a) and magnetism (FIG.
6b).
DETAILED DESCRIPTION OF THE INVENTION
[0037] In this context, the term "nanofibrillar cellulose" or
"nanofibrillated cellulose" (NFC for short) is used, it being
understood that also "microfibrillar cellulose" (MFC) or
"nanocellulose" are commonly used terms for the substance to be
described in more detail below.
[0038] The nanofibrillar cellulose consists of cellulose fibres
whose diameter is in the submicron range. It forms a self-assembled
hydrogel network even at low concentrations. At very low
concentration a viscous fluid is obtained. These gels of
nanofibrillar cellulose are highly shear thinning and thixotrophic
in nature.
[0039] The nanofibrillar cellulose is prepared normally from
cellulose raw material of plant origin. The raw material can be
based on any plant material that contains cellulose. The raw
material can also be derived from certain bacterial fermentation
processes. Plant material may be wood. Wood can be from softwood
tree such as spruce, pine, fir, larch, douglas-fir or hemlock, or
from hardwood tree such as birch, aspen, poplar, alder, eucalyptus
or acacia, or from a mixture of softwoods and hardwoods. Non-wood
material can be from agricultural residues, grasses or other plant
substances such as straw, leaves, bark, seeds, hulls, flowers,
vegetables or fruits from cotton, corn, wheat, oat, rye, barley,
rice, flax, hemp, manila hemp, sisal hemp, jute, ramie, kenaf,
bagasse, bamboo or reed. The cellulose raw material could be also
derived from cellulose-producing micro-organisms. The
micro-organisms can be of the genus Acetobacter, Agrobacterium,
Rhizobium, Pseudomonas or Alcaligenes, preferably of the genus
Acetobacter and more preferably of the species Acetobacter xylinum
or Acetobacter pasteurianus.
[0040] The term "nanofibrillar cellulose" refers to a collection of
isolated cellulose microfibrils or microfibril bundles derived from
cellulose raw material. Microfibrils have typically high aspect
ratio: the length might exceed one micrometer while the
number-average diameter is typically below 200 nm. The diameter of
microfibril bundles can also be larger but generally less than 1
.mu.m. The smallest microfibrils are similar to so called
elementary fibrils, which are typically 2-12 nm in diameter. The
dimensions of the fibrils or fibril bundles are dependent on raw
material and disintegration method. The nanofibrillar cellulose may
also contain some hemicelluloses; the amount is dependent on the
plant source. Mechanical disintegration of microfibrillar cellulose
from cellulose raw material, cellulose pulp, or refined pulp is
carried out with suitable equipment such as a refiner, grinder,
homogenizer, colloider, friction grinder, ultrasound sonicator,
fluidizer such as microfluidizer, macrofluidizer or fluidizer-type
homogenizer. In this case the nanofibrillar cellulose is obtained
through disintegration of plant celluose material and can be called
"nanofibrillated cellulose".
[0041] "Nanofibrillar cellulose" can also be directly isolated from
certain fermentation processes. The cellulose-producing
micro-organism of the present invention may be of the genus
Acetobacter, Agrobacterium, Rhizobium, Pseudomonas or Alcaligenes,
preferably of the genus Acetobacter and more preferably of the
species Acetobacter xylinum or Acetobacter pasteurianus.
[0042] "Nanofibrillar cellulose" can also be any chemically or
physically modified derivate of cellulose nanofibrils or nanofibril
bundles. The chemical modification could be based for example on
carboxymethylation, oxidation, esterification, or etherification
reaction of cellulose molecules. Modification could also be
realized by physical adsorption of anionic, cationic, or non-ionic
substances or any combination of these on cellulose surface. The
described modification can be carried out before, after, or during
the production of microfibrillar cellulose.
[0043] The nanofibrillated cellulose is according to one embodiment
non-parenchymal cellulose. The non-parenchymal nanofibrillated
cellulose may be in this case cellulose produced directly by
micro-organisms in a fermentation process or cellulose originating
in non-parenchymal plant tissue, such as tissue composed of cells
with thick, secondary cell wall. Fibres are one example of such
tissue.
[0044] The nanofibrillated cellulose can be made of cellulose which
is chemically premodified to make it more labile. The starting
material of this kind of nanofibrillated cellulose is labile
cellulose pulp or cellulose raw material, which results from
certain modifications of cellulose raw material or cellulose pulp.
For example N-oxyl mediated oxidation (e.g.
2,2,6,6-tetramethyl-1-piperidine N-oxide) leads to very labile
cellulose material, which is easy to disintegrate to microfibrillar
cellulose. For example patent applications WO 09/084,566 and JP
20070340371 disclose such modifications.
[0045] A specific form of nanofibrillated cellulose consists of
rod-like fibrils that are obtained by strong acid hydrolysis.
[0046] The nanofibrillated cellulose is preferably made of plant
material. One alternative is to obtain the nanofibrils form
non-parenchymal plant material where the nanofibrils are obtained
from secondary cell walls. One abundant source of cellulose
nanofibrils is wood fibres. The nanofibrillated cellulose is
manufactured by homogenizing wood-derived fibrous raw material,
which may be chemical pulp. When NFC where the cellulose is
modified cellulose is manufactured from wood fibres, the cellulose
can be labilized by oxidation before the disintegration to
nanofibrils. The disintegration in some of the above-mentioned
equipments produces nanofibrils which have the diameter of only
some nanometers, which is 50 nm at the most and gives a clear
dispersion in water. The nanofibrils can be reduced to size where
the diameter of most of the fibrils is in the range of only 1-20 nm
only.
[0047] Particularly preferred cellulose material to be used in the
invention is cellulose derivative, where cellulose molecules in MFC
contain some additional functional groups compared with the
chemical structure of native cellulose. Such groups can be
carboxymethyl, aldehyde and/or carboxyl or quaternary ammonium.
This kind of MFC samples are obtained e.g. by fibrillation of
carboxymethylated, oxidated (N-oxyl mediated), or cationized
cellulose pulp, respectively. The modification can be performed
also after fibrillation. When a gel consisting of any of these
modified MFC grades is introduced in the extraction agent, the gel
remains more easily coherent than with a native cellulose based
gel.
[0048] Other substances can be used in the invention by adding them
at a suitable process step to the nanofibrillated cellulose. The
method of adding depends on the nature of the substance. If the
substance is water soluble or hydrophilic, it is added in the NFC
hydrogel before the hydrogel is brought into contact with the
extraction agent. If the substance is non-water soluble or
hydrophobic but soluble in extraction agent, it is added in the
extraction agent from where it can enter the NFC hydrogel.
[0049] The aforementioned substances include hydrophilic polymers
which are added to the NFC hydrogel and hydrophobic polymers which
are added to the extraction agent. Amphiphilic copolymers are a
special group of polymers which can be added either to the NFC
hydrogel or to the extraction agent, depending on which copolymer
moiety is used for the hydrophobic/hydrophilic interaction with the
extraction agent or with the water of the hydrogel.
[0050] Some non-limiting examples of hydrophilic polymers that can
be added to the hydrogel so that they will be contained in the
fibrous products are chitosan and hydroxyethylcellulose.
[0051] If amphiphilic copolymers are used, they are used preferably
in form of micelles assembled from individual copolymer molecules.
The hydrophobicity or hydrophilicity is determined by the
shell-forming moiety of the copolymer. These micelles can be used
for encapsulating another substance, for example a drug.
[0052] However, in principle any substances having either
water-soluble molecules or non-water soluble molecules can be used,
provided that they are compatible with the NFC and other substances
used. The invention is not limited exclusively to the use of
polymers.
[0053] One special group of materials that can be included in the
fibrous products is active compounds which provide the product with
functionality of some kind. The active compound can be added either
to the NFC hydrogel or to the extraction agent, depending on their
characteristics.
[0054] For a particular application in medical field, the
additional material can be an active compound or mixture of
compounds which has pharmaceutical or biological activity, for
example a drug, an extra cellular matrix component, serum, growth
factor or protein. This makes it possible to use the product as
fibrous 1-, 2- or 3-dimensional structure which will release the
agent when placed in contact with a human body in external or
internal use, for example for structures to be placed on skin or to
be implanted subcutaneously. The aforementioned products could be
utilized also in vitro cell culture applications.
[0055] The substance added may be comprise reactive molecules which
will be chemically altered during the formation process of the NFC
product. The substance can for example be polymer precursors such
as monomers or oligomers which are polymerized in the NFC. The
reactive molecules can be added in the NFC hydrogel or in the
extraction agent.
[0056] Furthermore, the material to be added in the hydrogel can be
in particle form. If the hydrogel is to be introduced to the
extraction agent through orifices, the particle size must be
sufficiently small not to clog the orifices.
[0057] If the material is added to the extraction agent it will
penetrate the physical entities of NFC which are being formed in
the volume of the extraction agent. If one or more subsequent
volumes of extraction agents are used after the first volume, the
material can be added to such a subsequent volume from where it can
be penetrate the physical entities.
[0058] The dried NFC product can be aftertreated, for example
fibrous materials can be surface-modified to increase their
resistance to environmental factors, such as moisture. The dried
NFC product can also be impregnated with a substance, since it
contains porosity. Impregnation can be done for example by dipping
the product in a solution where the substance is dissolved and
drying. For example polymers can be introduced in this step in a
solvent, or polymer precursors such as monomers or oligomers can be
introduced and the polymerization can be conducted within the
product.
[0059] FIG. 1a shows the basic principle of a process that can be
applied in industrial scale. Aqueous NFC gel, hydrogel, is
introduced into a volume of organic extraction agent. The organic
extraction agent used is miscible with water. The extraction agent
can also be a mixture of chemically different extraction agents. In
the volume, the water in the hydrogel is completely or partly
exchanged with the extraction agent, that is, the water is
extracted out of the gel and it is gradually replaced with the
extraction agent. The exchange does not necessarily proceed to
completion because of the equilibrium conditions, but, as a rule,
the major part of the water which would make the drying difficult
is extracted in this step. Thus, the volume of the extraction agent
can be called an "extraction bath" for the NFC gel.
[0060] In industrial scale, the NFC gel is introduced into the
extraction bath through a suitable port that allows the formation
of discrete physical entities whose shape is determined by the port
and the rate of introduction of the gel. The port may comprise
several orifices through which the NFC hydrogel is extruded. The
hydrogel can be introduced for example through an extruder with a
suitable breaker plate that generates numerous elongate entities,
hydrogel "worms" or ribbons, in general thread-like elongate
objects in the extraction bath. Alternatively, a spray nozzle could
be used if the aim is to obtain small spheres or beads, in which
case the hydrogel is introduced at short intervals, "dropwise",
rather than as continuous strand. A port introducing the hydrogel
in this way can comprise several spray nozzles in parallel from
which the hydrogel issues as drops.
[0061] Its is also possible to introduce the hydrogel on larger
blocks into the extraction agent volume without any specific port.
In this case the blocks of hydrogel are crumbled mechanically in
the extraction agent to smaller pieces, for example using the
agitator blades, to enable the extraction agent exchange to start.
This alternative is used if the purpose is to make NFC raw material
for subsequent processes.
[0062] The concentration of the NFC fibrils in the gel is
preferably but not limited to 0.5-5%, based on the total weight of
the gel.
[0063] The extraction agent exchange process comprises preferably
two or more steps. After the gel has been introduced into the
extraction agent volume and the change of water with the extraction
agent proceeds to a certain equilibrium state which depends on the
relative amounts of the gel and extraction agent. Thereafter the
physical entities are separated from the extraction agent volume
and put into another volume of extraction agent to extract residual
water from the entities if the water content is needed to be
further lowered. The second extraction agent volume can consist of
different organic liquid than the first extraction agent volume.
The physical entities are, because of their size, easily separable
form each extraction agent volume by decantation or any other
separation technique, for example by filtering using a fine
mesh.
[0064] The non-water soluble molecule, e.g. polymer or monomer or
oligomer or active compound which is to be incorporated to the NFC
entities formed through the extraction agent is added to the second
extraction agent volume, if such step exists, or to the first
volume only, or both to the first volume and second volume of
extraction agent.
[0065] The extraction agent volumes which contain the extracted
water of the hydrogel are regenerated by distillation, where water
is separated, and the regenerated extraction agent can be
recirculated back to the extraction agent exchange process.
[0066] After the extraction agent exchange process the physical
entities are dried by allowing the extraction agent to evaporate
from the entities. The physical entities may contain still some
residual water, which evaporates easily without interfering with
the drying process. The drying is preferably performed at elevated
temperature or lower temperature by evaporation and/or vacuum
(reduced pressure). The organic extraction agent released from the
entities is collected, condensed and recirculated back to the
extraction agent exchange process. During the drying the entities
shrink from their original dimensions.
[0067] FIG. 1b shows schematically a process where continuous
threads or filaments can be produced. The NFC hydrogel is extruded
through several orifices to the volume of organic extracting agent
where it is allowed to undergo the solvent exchange process during
a predetermined residence time. Rollers can be used to guide the
threads or filaments inside the volume of extracting agent so that
the residence time is accomplished. The threads or filaments that
are drawn out of the volume are dried in hot gas flow to evaporate
the extraction agent and residual water and collected subsequently
on a reel. In this process the threads can also be drawn when wet
in a predetermined draw ratio (stretching), which increases the
tensile strength. The volume of the extracting agent can contain a
substance that is to be incorporated in the filaments or threads
and/or the NFC hydrogel may contain a substance for the same
purpose. Alternatively, a substance can be incorporated in the
filaments or threads after drying. These substances were mentioned
above and will also be discussed later in more detail.
[0068] The organic extraction agent can be also removed by pressing
the elongated thread-like NFC gel entities between two rolling
cylinders that compress the wet fiber to ribbon-like filament. This
can be done in the process of FIG. 1b or in any other process where
elongate thread-like NFC gel entities are produced. The mechanical
pressing can precede the final drying stage.
[0069] After drying the physical entities, comprising the NFC and
any substances incorporated in it through the hydrogel and/or an
extraction agent volume, can be comminuted by a suitable mechanical
process to a final particle size. This can be done to fiber-like
structures as well, if they are not to be used as such in
subsequent constructions.
[0070] FIG. 2 is an exemplary graph which shows how the water
content of ethanol (the extraction agent) increases in course of
the extrusion of an aqueous N-oxyl-mediated oxidated NFC gel into
an ethanol bath, in proportion of one part gel/four parts ethanol
(vol/vol). Both "crumb" (adding the hydrogel in blocks which were
crumbled mechanically) and extrusion methods were used (marked
"Non-extruded" and "Extruded"). The solvent volume was stirred in
both cases during the introduction. The exchange of water and
ethanol is driven by a concentration gradient such that
concentration equilibrium is reached in both the NFC gel matrix and
the solvent medium. At this point the amount of water in the
ethanol bath should equal the amount of water within the gel. The
water content of the ethanol bath at different times was determined
with Karl Fischer titration of samples taken at different times.
The figure shows how how the mixture reaches the equilibrium
plateau already after 20 minutes and the majority of water has
diffused out of the gel entities. The figure also shows how the
solvent exchange begins immediately after the aqueous gel is
exposed to the solvent and the solvent exchange proceeds quickly
during the 10 first minutes.
[0071] The detailed fiber preparation is first illustrated using
NFC material prepared via TEMPO-mediated oxidation of wood pulp and
subsequent homogenization in a microfluidizer, yielding high-aspect
ratio nanofibrils with diameters down to a few nanometer and length
up to several micrometers. These nanofibers form strong hydrogels
in water. Scanning force microscopy (SFM) in FIG. 3b shows
individual NFC nanofibrils, where the section analysis reveals
heights in the range of 1-2 nm.
[0072] The generation of macroscopic NFC fibers can be accomplished
via simple wet extrusion of the hydrogels (c.apprxeq.1 wt %) into
the coagulation bath containing an organic water soluble extraction
agent (e.g. ethanol, dioxane, isopropanol, THF or the like, FIG.
3a). A dye-labeled NFC dispersion clearly visualizes the formation
of intact NFC threads in the photograph. The coagulation bath
induces almost instantaneously a skin formation on the outside of
the extruded NFC hydrogel dispersion, imparting the fibers with a
sufficient mechanical stability. The resulting NFC fibers within
the coagulation bath are stabilized against inter-fiber aggregation
and can be easily manipulated. Further on, essentially complete
extraction agent exchange can be accomplished, if desired, via
diffusion inside the coagulation bath. Afterwards the fibers are
simply dried in air or at higher temperature. A complete extraction
agent exchange is not necessary to create stable fibers during
drying. Similar fiber structures were obtained from
carboxymethylated MFC and cationized MFC.
[0073] Wet extrusion easily allows the preparation of long fibers,
and non-wovens or fiber mats are accessible via geometrically
controlled extrusion patterns. FIG. 4 displays a 1 m long fiber and
a defined, non-woven fiber mat. Arbitrary structures could be
created by suitable 2D or 3D printing techniques. The diameter of
the resultant fibers can principally be adjusted by the size of the
needle or die used for the process. The cross-sectional diameter of
the herewith-prepared threads is not perfectly circular and may
vary slightly along the thread. These imperfections originate from
the non-uniformities (small aggregates etc.) present within the
hydrogel of the strongly hydrogen-bonding NFC nanofibrils. The
white appearance of the NFC fibers points to a certain porosity and
light scattering at internal NFC/air interfaces.
[0074] In order to assess the microscopic order of the individual
nanofibrils within the macroscopic fibers, we used scanning
electron microscopy (SEM) to reveal details down to the nanoscale
(FIG. 4e-g). A preferential orientation on the micron-scale can be
identified as induced by the extrusion process. However, the
individual NFC nanofibers only show a moderate orientation, as
similarly indicated by the modest birefringence when observing the
thread between crossed polarizers (FIG. 4d&g,h). The shear
forces created during the extrusion are obviously insufficient to
allow for a complete alignment of such long and entangled
nanofibrils. The threads exhibit a nanoporous structure with pores
in the size of up to 25 nm, according to the SEM imaging. Comparing
the diameter of the resulting fiber with the dimensions of the
extrusion needle and the known concentration yields a porosity
below 10%. Although porosity may in the first instance not be
beneficial for the mechanical properties, it can be turned into an
advantage for incorporating functionalities as discussed above.
[0075] A stress-strain curve of an individual NFC thread is shown
in FIG. 5a. Although the individual NFC nanofibrils do not exhibit
a global alignment, the mechanical properties, as obtained by
tensile tests, already demonstrate impressive values. Typical
values for stiffness and ultimate strength at break are 22.5 GPa
and 275 MPa (at 4% strain maximum elongation, FIG. 5a),
respectively. These mechanical characteristics are slightly larger
than the ones typically obtained for non-infiltrated NFC
nanopapers. The orientation of the nanofibrils within the threads,
even if not very pronounced, may be responsible for the better
performance. The stiffness of the NFC threads is around one third
to one fourth of glass fibers and Kevlar 29, respectively, and is
twice as high than some of the best spider silk grades. It is also
an order of magnitude larger than typical Rayon fibers, which are
based on regenerated cellulose.
[0076] The area under the stress strain curves allows us to
estimate the work of fracture to a high value of 7.9 MJ/m.sup.3.
Such a mechanical performance, combining stiffness, strength and
toughness, is not frequently found, especially among slightly
porous materials. These properties render the NFC fibers, prepared
with a surprisingly simple room temperature procedure, a most
promising source for high-performance and biodegradable
fiber-reinforced composite materials.
[0077] Beyond excellent mechanical properties, we demonstrate how
to equip the NFC threads with attractive multifunctional
properties, that may for instance allow the manipulation in
external fields, produce optically transparent fiber-reinforced
composites, enable damage sensing under stress and controlled
release of active compounds. These goals need to be met if we want
to approach applications such as actuators, in biomedicine,
electronic or optic devices, or for high-performance lightweight
(bio)composite materials.
[0078] Optical transparency and tailored stability in outdoor
environments are prime goals with respect to future biobased
composites. FIG. 5b shows how opaque NFC threads can be rendered
translucent by infiltration of the NFC fibers with a resin,
matching the refractive index of cellulose, and subsequent
polymerization. The refractive index of cellulose fibers is 1.62
and 1.54 in the longitudinal and transverse direction,
respectively. This matches well to the refractive index of an
acrylic resin based on tricyclodecane dimethanol dimethacrylate
(RI=1.53). The impregnation replaces interfaces of high-refractive
index contrast between NFC nanofibrils and air and henceforth leads
to a strong reduction of light scattering, resulting in nearly
fully transparent, biobased filaments (FIG. 5b). As a logic
consequence, fiber-reinforced plastics based on the same resin and
NFC threads will also be highly translucent. Such transparent
materials are for instance out of reach for carbon-fiber reinforced
materials due to the inherent light adsorption of the carbon
fibers.
[0079] Secondly, real-life outdoor applications of fibers (fiber
reinforced plastics or concrete, textiles etc.) require a control
of the long-term stability and enhanced durability. Again, we are
able to meet this criteria via simple modifications. A facile
gas-phase mediated (chemical vapor) deposition of a perfluorinated
trichlorosilane, which is a model compound for typical
hydrophobization agents, onto the reactive hydroxy functions of the
glucose units leads to a significant change in wettability. FIG. 5c
shows NFC fibers after swelling in water for four hours before
(left) and after (right) surface modification with a fluorinated
silane. The water up-take is drastically reduced, thus minimizing
both a mechanical softening as well as the available surface for
microbial attacks in humid or even wet environments. Note that this
modification also demonstrates the straight-forward conjugation of
functional entities, e.g. for sensing or recognition, to the large
amount of hydroxyl groups present on the cellulose surface.
[0080] Incorporation of functional compounds is however not
restricted to chemical binding. FIG. 3a already sketches some of
the possible pathways of how to include active molecules (such as
drugs or antibiotic agents), polymers for composite fibers or
inorganic materials (carbon nanotubes, inorganic nanoparticles)
into the NFC threads. FIG. 5d demonstrates results for
incorporating hydrophilic and hydrophobic model compounds (i.e.
dyes) into the NFC threads via physical entrapment. It shows NFC
threads loaded with hydrophobic nile red (A) and hydrophilic
rhodamine 6G (B). Different release characteristics for hydrophobic
nile red (C), cationic rhodamine 6G (D) and anionic sulforhodamine
(E) are shown. The arrows indicate the swollen NFC thread in (E).
If the desired materials (e.g. rhodamine derivatives) are
water-soluble, they can simply be premixed into the NFC hydrogel
and co-extruded into the fiber. Depending on the interaction with
the slightly anionic NFC, the dopants remain tightly entrapped
within the NFC thread. Since only very short ageing times are
necessary to obtain sufficient mechanical stability for drying,
excessive release of entrapped material can be prevented on this
timescale. Hence, the encapsulation is near quantitative. On the
other hand, hydrophobic compounds can preferentially be infused
into the NFC fibers via prolonged extraction agent exposure and
full exchange in the coagulation bath. The final concentration is
obviously limited to the concentration of the compound in the
coagulation bath if no attractive interactions between NFC and the
compound exist. Herein, the hydrophobic dye nile red serves as an
example. Both fibers with encapsulated hydrophilic and hydrophobic
model compounds are strongly colored, confirming the presence of
significant quantities of materials.
[0081] Due to the large versatility with respect to the polarity of
the dopants, it is possible to virtually encapsulate any kind of
compound into the fibers. The complete preparation process can thus
be considered a platform technology for robust, functional and
payload-containing biofibers. Interestingly, depending on the
interaction of the entrapped compounds with the NFC nanofibril
(steric, ionic and hydrogen bonding), different release profiles
can be achieved. A strong difference between the different net
charges of the rhodamine derivatives can be observed. Whereas the
anionically charged sulforhodamine diffuses out of the fiber, the
cationic Rhodamine 6G resides nicely entrapped within the NFC
threads. This behavior originates from the tighter electrostatic
binding of Rhodamine 6G with the slightly anionic NFC. Similarly,
the hydrophobic compound, infused via the extraction agent bath,
remains essentially within the NFC thread in the observed time
frame (FIG. 5d). We expect that the release kinetics can be widely
tuned by tailoring the charge and hydrogen-bonding interaction,
embedding the active compounds into coextruded drug carriers
(liposome or micelles) or attaching them to polymers or even
directly onto the nanofibrils.
[0082] Electric conductivity can be achieved by impregnating the
dried fibers in a toluene solution containing doped polyaniline
(PANI). PANI is one of few commercially available conducting
polymers. The undoped emeraldine base form of PANI can be rendered
electrically conducting via complexation with strong acids, leading
to an emeraldine salt. To achieve sufficient solubility, suitable
plasticizing organic acids, such as dodecyl benzene sulfonic acid
(DBSA), are used. Herein, we use a mixture with a excess of the
doping acid. The impregnation can be accomplished via simple
dipping of the NFC filaments into a solution of PANI(DBSA).sub.1.1
(FIG. 6a). Due to the high surface energy and the beneficial
porosity, the conducting polymer tightly adsorbs onto and into the
fibers. Rinsing with copious amounts of toluene to remove any
unbound polymer results in a slightly green NFC fiber, where the
color originates from the remaining adsorbed PANI(DBSA).sub.1.1.
Elemental analysis of the nitrogen content reveals a total content
of adsorbed PANI(DBSA).sub.1.1 of around 13.1 wt %. The final NFC
filaments exhibit a conductivity as high as 1.910.sup.-3 S/cm. This
value is remarkably high considering the only small weight fraction
of adsorbed material. FIG. 4aD shows the linear current-voltage
curve demonstrating the ohmic behavior of the conducting
PANI-modified NFC fibers. This current voltage diagram reveals an
ohmic behavior, demonstrating a structurally intact conducting
pathway along the fibers.
[0083] This immobilization process is not restricted to a post
impregnation of dried fibers, but could already be realized by
using a PANI(DBSA).sub.1.1 doped extraction agent bath. This simple
in-situ functionalization into conducting biofibers can find
widespread interest and is of direct relevance for applications in
(bio)electronics, sensing and for the damage monitoring of
fiber-reinforced composite materials.
[0084] In the last section, we will demonstrate NFC threads that
are susceptible to actuation in external fields. For this purpose,
we prepared inorganic/organic hybrid fibers with ferromagnetic
cobalt ferrite nanoparticles attached to the threads. The
CoFe.sub.2O.sub.4 particles are synthesized via a simple aqueous
coprecipitation reaction of FeSO.sub.4 and CoCl.sub.2 (FIG. 6bF) in
the presence of an already prepared NFC thread. During this
process, the CoFe.sub.2O.sub.4 particles easily attach to the NFC
thread and remain tightly trapped after drying, resulting in a dark
brown fiber (FIG. 6bG). The SEM images reveal clustered magnetic
nanoparticles with a size distribution centered at diameters of
50-90 nm, whose weight fraction can be estimated to 16.7 wt %
according to thermogravimetric analysis (FIG. 6bH,I).
Interestingly, the second and third degradation steps of pure NFC
at 600-720.degree. C. are shifted to around 800.degree. C.,
indicating a close interaction between both components. The
magnetization loop curve, as determined by a SQUID magnetometer at
300.degree. C., reveals a saturation moment of the modified fiber
of 6.5 emu/g (FIG. 6bK). Compared to the saturation moment of bulk
CoFe.sub.2O.sub.4 (80 emu/g) yields the mass concentration of
magnetically active CoFe.sub.2O.sub.4 to be 8.1 wt %. The
difference in comparison to the value obtained by TGA can be
explained by surface effects on nanoparticles and incomplete
crystallization of the ferrite, which are known to decrease the
saturation magnetization.
[0085] The resulting hybrid filaments can be easily manipulated
with a weak fingertip-sized household magnet (FIG. 6bL). Due to the
small coercivity, the threads can be classified as soft magnets.
Therefore, we envisage applications in magnetically-actuable
biocomposites and fiber materials as well as for components in
intelligent textiles, for magnetic shielding and in microwave
technology.
[0086] The process can be modified from the above-described process
within the scope of the invention. There can be only one extraction
agent-exchange step where water within the gel is exchanged with
the extraction agent, or the number of steps can be two or more.
Further, instead of forming continuous elongated threads or ribbons
from the gel in the extraction agent-exchange process, the physical
entities formed of the gel when it is introduced in the extraction
agent volume can take the shape of flakes, beads, etc., depending
on the method of supply, for example introduction rate combined
with the port structure (orifices), through which the hydrogel
issues and which determines the shape of the physical entities. The
physical entities formed of the gel in the extraction agent volume
should be easily separable form the volume by simple methods such
as decantation, lifting, skimming, sedimentation, filtering through
a fine mesh etc.
[0087] The port can be immersed in the volume of organic extraction
agent, in which case the hydrogel comes in contact with the organic
extraction agent immediately after it has issued from the port, or
it can be separated from the volume, in which case the hydrogel
enters air for a short while before coming in contact with the
organic extraction agent.
[0088] As was explained in more detail earlier, the hydrogel
serving as the raw material can, besides NFC and water and the
substances mentioned hereinabove, also contain other substances,
which can be dissolved or dispersed in the gel. It is also possible
that the NFC contained in the gel and consequently in the fiber
product or composite may be a blend of various chemically different
NFC grades.
[0089] The organic extraction agent used in the extraction agent
volumes of two or several subsequent extraction agent-exchange
steps need not necessarily be the same extraction agent chemically,
provided that all extraction agents used are miscible with water.
However, in view of the simplicity of the process and recovery and
recycling of the extraction agent, the same extraction agent is
preferred in all steps.
[0090] The dried NFC product can be in fibrous form in a
1-dimensional structure (thread or filament) or 2- or 3-dimensional
structure (threads or filaments laid together, possibly interlocked
mechanically using known textile bonds or deposited as non-woven
webs).
[0091] The dried product can also exist also as film, in which case
its manufacturing stages includes introducing the NFC hydrogel
through a narrow slit-like orifice into the extraction agent bath
where the exchange of solvents and subsequent drying stages occur
analogously with fibrous products, possible additional constituents
being added already in the hydrogel, through an extraction agent
bath, or in a post-treatment.
* * * * *